An organic electroluminescent device is a device emitting light while electrons and holes disappear after being coupled in pairs when an electron charge is injected into an organic film formed between an electron injection electrode (cathode) and a hole injection electrode (anode). The organic electroluminescent device has advantages in that the device may be formed on a flexible transparent substrate made of, for example, plastic material, driven at a low voltage (10V or less) as compared with a plasma display panel or an inorganic electroluminescent (EL) display, has relatively low power consumption and excellent color sensation.
Generally, the organic electroluminescent device has a structure consisting of a substrate, an anode, a hole injection layer receiving a hole from the anode, a hole transport layer transporting the hole, a luminescent layer emitting light while the hole and an electron are bonded, an electron transport layer receiving the electrode from a cathode to transport the electron to the luminescent layer, and the cathode. In some cases, the luminescent layer may be configured by applying a small amount of a fluorescent or phosphorescent dye to the electron transport layer or the hole transport layer without a separate luminescent layer. In the case of using a polymer, generally, one polymer may entirely serve as the hole transport layer, the luminescent layer, and the electron transport layer. Organic thin film layers between two electrodes may be formed by a method such as a vacuum deposition method, a spin coating method, an inkjet printing method, a roll coating method, or the like, and for effective injection of the electron from the cathode, a separate electron injection layer may be inserted.
In the case in which an interface between the electrode and an organic material is stabilized, or in the case of the organic material, since there is a large difference in a movement rate between the hole and the electron, an appropriate hole transport layer and electron transport layer are used, the hole and the electron may be effectively transported to the luminescent layer. In addition, in order to balance the hole and the electron densities in the luminescent layer to increase luminescence efficiency, the organic electroluminescent device is manufactured to have a multi-layer thin film structure.
Meanwhile, as a representative example of the existing electron transport material, there are aluminum complex and beryllium complex such as Alq3 (tris(8-hydroxyquinoline)aluminum(III)) and Bebq (bis(10-hydroxybenzo-[h]quinolinato)beryllium). However, in the case in which these materials are used in a blue electroluminescent device, color purity may be decreased due to luminescence caused by exciton diffusion.
In addition, TPBI, which was reported by Kodak in 1996 disclosed in U.S. Pat. No. 5,645,948, (See the following structure), is known to be a representative material for electron transport layer having an imidazole group. This material contains three N-phenyl benzimidazole groups at 1, 3, and 5 substitution positions of benzene, and has a function of blocking holes from a luminescent layer as well as transporting electrons. However, stability of TPBI is too low to be actually used in the device.

In the case of the electron transport material according to the related art, unlike the reported contents, actually, the material may slightly improve only a driving voltage, or there are problems such as significant deterioration of a device driving lifespan or negative properties such as variation in the device lifespan according to the color, and deterioration of thermal stability, or the like.
In addition, a fluorescent material was used in the organic electroluminescent device according to the related art, but gradually, phosphorescent material has been mainly used in the organic electroluminescent device. Therefore, in the electron transport material, which is a common material of the organic electroluminescent device, electron mobility appropriate for the phosphorescent material, low driving voltage, and a hole blocking property have been required.